This invention relates to surface-treated steel strips or sheets from which seam-welded cans are produced, and more particularly, to surface-treated steel strips or sheets having such improved weldability as to permit can bodies to be joined into food cans by electric resistance seam welding.
Among food can-forming materials there have been most wedely used tin-coated steel strips generally called tin plates. In order to join the mating edges of a can body, conventional soldering techniques were previously used. Because of the toxicity of lead contained in conventional solder, pure tin solder has recently become prevalent. The pure tin solder, however, has a technical problem in making a joint because of inferior wetability during the soldering process and is so expensive as to create the economic problem of increased manufacture cost.
On the other hand, in recent years, food containers have enjoyed the development of inexpensive, competitive materials such as polyethylene, aluminum, glass, processed paper and the like. Despite their significantly improved corrosion resistance among other advantages, tin plate cans having expensive tin thickly coated thereon to a coating weight of as great as 2.8 to 11.2 g/m.sup.2 require a relatively high cost of manufacture and have encounterd severe competition.
In order to overcome the above-described drawbacks of tinplate cans, electric resistance welding of can bodies has recently replaced the conventional soldering technique and become widespread. There is the need for can-forming steel compatible with electric resistance welding.
In addition to tinplate discussed above, tin-free steel of chromium type is another typical example of conventional can-forming steel. The tin-free steel is prepared by carrying out an electrolytic chromate treatment on steel to form a layer of metallic chromium and hydrated chromium oxides on the surface. Since the relatively thick hydrated chromium oxide film on the surface has a relatively high electric resistance, the chromated steel is ineffectively welded to form a weld of insufficient strength and thus unsuitable as welded can-forming steel despite its economic advantage.
Since other can-forming materials are also inadequate as welded can-forming material, a variety of proposals have been made. One example is nickel-plated steel, typically "Nickel-Lite" announced by National Steel Corporation of the U.S. which is prepared by plating a steel strip with nickel to a thickness of about 0.5 g/m.sup.2 followed by a conventional chromate treatment. Inferior adhesion of lacquer and inferior weldability in high speed welding at 30 m/min. or higher have limited the spread of this nickel-plated steel.
Another example is "Tin Alloy" announced by Jones & Laughlin Steel Corporation of the U.S. This is prepared by thinly coating a steel strip with tin to a thickness of about 0.6 g/m.sup.2 and effecting tin reflow or flow melting followed by a conventional chromate treatment. Unfortunately, rust resistance, lacquer adhesion and weldability are insufficient.
In general, can-forming steel sheets intended for electric resistance welding are required to exhibit improved weldability and corrosion resistance after lacquering. These requirements will be explained in detail. There must be an optimum welding electric current range within which a weld zone having sufficient weld strength is provided at the end of welding without any weld defects such as so-called "splashes". Since welded cans are filled with foodstuffs after lacquer coating, the underlying steel must have sufficient adhesion to lacquer to take full advantage of the corrosion prevention of the lacquer film. Furthermore, despite defects unavoidably occurring in a lacquer film, the improved corrosion resistance of the underlying steel itself must prevent corrosion from proceeding.